Originally published in the May 2014 issue of Sweeping Magazine
Question: I have a customer that wants me to paint be outside of their brick chimney. They say it leaks and gets wet in the attic when it's raining, and that it just looks old and they don't like it anymore. Is it okay to do this for them?
by Mike Segerstrom
Usually no. Chimneys have been painted for years. By the homeowner, painters, carpenters, chimney sweeps, the handyman, even the neighbor. Painting the chimney may temporarily prevent water from getting in, but it can do much more harm than good.
How many times have we seen a previously painted chimney, where not only is the paint peeling off, but the bricks underneath are spalling and damaged? And how many times have we seen a chimney painted, because the bricks were flaking apart and damaged? Where somebody thought painting would help? The biggest problem with painting a chimney is perhaps that it traps water and/or moisture in the masonry. The chimney can't breathe.
Water can get behind the paint through defects in the crown, poorly painted areas, and even through holes so small we can't even see them. Over time this water can freeze, which can damage the brick/ masonry and the paint. The paint will begin to fail and develop larger holes and cracks, and the masonry will continue to deteriorate.
The other potential for moisture damage is from flue gas condensation. If a chimney is unlined, or lined with clay that has cracks and voids, water vapor from flue gas condensation can migrate outside of the flue into the chimney structure. If this moisture migrates far enough away from the flue, it can also freeze and result in the same type of damage.
This freezing damage is in addition to the common damage we might expect to see when masonry is exposed to acidic flue-gas condensation and excessive water penetration. Painting a chimney exterior can add to these conditions if they are already present, or create new damage.
So in most cases, applying a breathable water repellent, instead of paint or a waterproofing sealer that can't breathe, is the best choice. This will allow the masonry to breathe, and let water or moisture out. When considering applying a water-repellent, the chimney should be repaired if necessary first. This includes repairing the crown and any exterior brick, mortar or masonry. The flashing should also be checked and repaired or replaced if necessary.
Once the chimney is repaired, applying a water repellent can help preserve the original masonry, and help preserve any repaired areas. Exterior repairs won't prevent flue gas condensation migration, so the liner(s) should be checked to ensure that it properly contains any condensation that may occur. And of course a rain cap should be present on the flue(s)!
If all the chimney flues are lined with water tight liners, rain caps are present on all of the flues and the crown is water tight, then painting the chimney exterior may be considered. But even if these conditions are met, the chimney may also require weep holes for ventilation (depending on the size and type of chimney) and would still need to be evaluated routinely. In this situation, there is still the potential for water penetration, and some subsequent damage.
Before considering painting as an option, consider repairing and cleaning the chimney exterior, and applying a water-repellent. Properly applying a water repellent will help protect the chimney, and does not cause any known damage to the chimney. Applying waterproof paint or non-breathable sealants can.